Capacitatively shunted quadrifilar helix antenna

ABSTRACT

A quadrifilar helix antenna is provided having a feedpoint for the antenna connecting to individual helical antenna elements. A capacitive network, distributed along the length of the antenna, constitutes a variable frequency shunting network. At each position a first capacitive structure, that may comprise a single capacitor or multiple capacitors in series, interconnects a first pair of opposite antenna elements; a second capacitive structure interconnects the second pair of opposite antenna elements. As an applied frequency increases, the capacitive structures progressively short the opposite antenna elements thereby electrically reducing the antenna length.

STATEMENT OF GOVERNMENT INTEREST

The invention described herein may be manufactured and used by or forthe Government of the United States of America for governmental purposeswithout the payment of any royalties thereon or therefor.

CROSS REFERENCE TO RELATED APPLICATION

U.S. patent Ser. No. 09/356,803 filed Jul. 19, 1999 by the inventorhereof and assigned to the assignee hereof is incorporated herein byreference.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

This invention generally relates to antennas and more specifically toquadrifilar antennas.

(2) Description of the Prior Art

Numerous communication networks utilize omnidirectional antenna systemsto establish communications between various stations in the network. Insome networks one or more stations may be mobile while others may befixed land-based or satellite stations. Antenna systems that areomnidirectional in a horizontal plane are preferred in such applicationsbecause alternative highly directional antenna systems become difficultto apply, particularly at a mobile station that may communicate withboth fixed land-based and satellite stations. In such applications it isdesirable to provide a horizontally omnidirectional antenna system thatis compact yet characterized by a wide bandwidth and a goodfront-to-back ratio in elevation with either horizontal or verticalpolarization.

Some prior art omnidirectional antenna systems use an end fedquadrifilar helix antenna for satellite communication and a co-mounteddipole antenna for land based communications. However, each antenna hasa limited bandwidth. Collectively their performance can be dependentupon antenna position relative to a ground plane. The dipole antenna hasno front-to-back ratio and thus its performance can be severely degradedby heavy reflections when the antenna is mounted on a ship, particularlyover low elevation angles. These co-mounted antennas also have spatialrequirements that can limit their use in confined areas aboard ships orsimilar mobile stations.

The following patents disclose helical antennas that exhibit some, butnot all, the previously described desirable characteristics:

U.S. Pat. No. 5,485,170 (1996) to McCarrick discloses a mobile satellitecommunications system (SMAT) mast antenna with reduced frequencyscanning for mobile use in accessing stationary geosynchronous and/orgeostable satellites. The antenna includes a multi-turn quadrifilarhelix antenna that is fed in phase rotation at its base and is providedwith a pitch and/or diameter adjustment for the helix elements, causingbeam scanning in the elevation plane while remaining relativelyomni-directional in azimuth. The antenna diameter and helical pitch areoptimized to reduce the frequency scanning effect, and a technique isdisclosed for aiming the antenna to compensate for any remainingfrequency scanning effect.

U.S. Pat. No. 5,701,130 (1997) to Thill et al. discloses a self phasedantenna element with a dielectric. The antenna element has two pairs ofarms in a crossed relationship to transceive a signal at a resonantfrequency. A dielectric is disposed adjacent an arm to obtain a selfphased relationship in the arms at the resonant frequency. The arms canform crossed loops or twisted crossed loops such as a quadrifilar helixantenna element. A dielectric collar on arms of the same loop causescurrents to be equally spaced from one another. The antenna size isreduced and a cross section of the antenna element appears circularwithout degradation of a gain pattern when the dielectric is used on acertain arm.

In U.S. Pat. No. 5,721,557 (1998) Wheeler et al. disclose a nonsquintingend-fed quadrifilar helix antenna. Each conductor of the antenna is fedwith a successively delayed phase representation of the input signal tooptimize transmission characteristics. Each of the conductors isseparated into a number, Z, of discrete conductor portions by Z−1capacitive discontinuities. The addition of the capacitivediscontinuities results in the formation of an antenna array. The endresult of the antenna array is a quadrifilar helix antenna which isnonsquinting, that is, the antenna radiates in a given directionindependently of frequency.

There exists a family of quadrifilar helixes that are broadbandimpedance wise above a certain “cut-in” frequency, and thus are usefulfor wideband satellite communications including Demand Assigned MultipleAccess (DAMA) UHF functions in the range of 240 to 320 MHz and for othersatellite communications functions in the range of 320 to 410 MHz).Typically these antennas have (1) a pitch angle of the elements on thehelix cylindrical surface from 50 down to roughly 20 degrees, (2)elements that are at least roughly ¾ wavelengths long, and (3) a“cut-in” frequency roughly corresponding to a frequency at which awavelength is twice the length of one turn of the antenna element. Thisdependence changes with pitch angle. Above the “cut-in” frequency, thehelix has an approximately flat VSWR around 2:1 or less (about the Z₀value of the antenna). Thus the antenna is broadband impedance-wiseabove the cut-in frequency. The previous three dimensions translate intoa helix diameter of 0.1 to 0.2 wavelengths at the cut-in frequency.

For pitch angles of approximately 30° to 50°, such antennas provide goodcardioid shaped patterns for satellite communications. Good circularpolarization exists down to the horizon since the antenna is greaterthan 1.5 wavelengths long (2 elements constitute one array of the dualarray, quadrifilar antenna) and is at least one turn. At the cut-infrequency, lower angled helixes have sharper patterns. As frequencyincreases, patterns start to flatten overhead and spread out near thehorizon and small nulls start to form overhead. For a given satelliteband to be covered, a tradeoff can be chosen on how sharp the pattern isallowed to be at the bottom of the band and how much it can be spreadout by the time the top of the band is reached. This tradeoff is made bychoosing where the band should start relative to the cut-in frequencyand the pitch angle.

For optimum front-to-back ratio performance, the bottom of the bandshould start at the cut-in frequency. This is because, for a givenelement thickness, backside radiation increases with frequency (thefront-to-back ratio decreases with frequency). This decrease offront-to-back ratio with frequency limits the antenna immunity tomultipath nulling effects.

Other factors that influence the front-to-back ratio include the methodof feeding the antenna, the physical size of antenna elements, thedielectric loading of the antenna elements and the termination of theantenna elements. Looking first at antenna feeding, the front-to-backratio improves when an antenna is fed in a “backfire mode” such that theantenna feed point is at the top of a vertically oriented antenna, asopposed to a “forward fire mode” when the feed point is at the bottom ofthe antenna.

Thinner elements increase the front-to-back ratio. However, as theelements become thinner, the input impedance to the antenna increasesand introduces a requirement for impedance matching. Alternatively,lower impedances can be obtained by constructing an antenna with apartial overlap of the antenna elements to increase capacitance.However, a loss of impedance bandwidth starts to occur since thecapacitance is a non-radiating capacitance; that is, no radiation canoccur from the overlapped areas of the antenna.

Increasing the dielectric loading of the helix elements decreases thefront-to-back ratio. Wide flat elements found in many helix antennashave a pronounced loading if one side of each antenna element touches adielectric, as in the case where the dielectric is a support cylinderfor the antenna. If the gap between adjacent elements is small, thefield is strongly concentrated in the gap and any dielectric in the gapwill load the antenna strongly. Quadrifilar helix antennas can terminatewith open or shorted ends remote from the feed point. It has been foundthat antennas with open ends have a slightly higher front-to-back ratiothan do antennas with shorted ends.

My above-identified pending U.S. patent Ser. No. 08/356,803 discloses anantenna having four constant-width antenna elements wrapped about theperiphery of a cylindrical support. This construction provides abroadband antenna with a bandwidth of 240 to at least 400 MHz and withan input impedance in a normal range, e.g., 100 ohms. This antenna alsoexhibits a good front-to-back ratio in both open-ended and shortedconfigurations. In this antenna, each antenna element has a widthcorresponding to about 95% of the available width for that element.However, it was found that this antenna could require a tradeoff betweenthe pattern shapes in the transmit and receive bands. It becamenecessary to allow patterns at lower receive frequencies to becomesharper overhead than desired. At higher transmit frequencies, it becamenecessary to accept overhead patterns that were flatter overhead thandesired. At even higher frequencies, nulls were observed in the patternsbecause the element lengths were becoming long enough electrically formultilobing to begin.

SUMMARY OF THE INVENTION

Therefore it is an object of this invention to provide a broadbandunidirectional hemispherical coverage radio frequency antenna.

Another object of this invention is to provide a broadbandunidirectional hemispherical coverage antenna with good front-to-backratio over a range of frequencies.

Yet still another object of this invention is to provide a broadbandunidirectional hemispherical coverage antenna that operates with acircular polarization and that exhibits a good front-to-back ratio.

Yet still another object of this invention is to provide a broadbandunidirectional hemispherical coverage antenna that provides anessentially constant radiation pattern over a range of frequencies.

In accordance with this invention the above objects are achieved by anantenna that extends along an antenna axis between a feed end and another end and that carries a plurality of pairs of diametrically opposedantenna elements wrapped helically about the support. Each antennaelement has a length determined by a cut-in frequency. A capacitivenetwork spans the antenna elements in each pair at correspondingpredetermined positions from the other end for shorting the pairs ofantenna elements at a characteristic frequency greater than the cut-infrequency.

In accordance with another aspect of this invention, a quadrifilar helixantenna operates over a frequency bandwidth defined by a minimumoperating frequency and extends along an antenna axis between first andsecond ends of the antenna. Four equiangularly spaced helical antennaelements extend along the support between the first and second end, eachantenna element has a length of at least ¾ wavelength at the minimumantenna operating frequency and has a substantially constant thicknessand width along its length. Each diametrically opposed set of elementsconstitutes an element pair whereby the antenna has first and secondpairs of antenna elements. A plurality of sets of capacitive elementsconnect between the antenna elements in each pair, each set beingconnected at a different position along the antenna axis and eachcapacitive element in a set connected to said respective antenna elementpair at the same position along the antenna axis.

BRIEF DESCRIPTION OF THE DRAWINGS

The appended claims particularly point out and distinctly claim thesubject matter of this invention. The various objects, advantages andnovel features of this invention will be more fully apparent from areading of the following detailed description in conjunction with theaccompanying drawings in which like reference numerals refer to likeparts, and in which:

FIG. 1 is a perspective view of one embodiment of a quadrifilar helixantenna constructed in accordance with this invention;

FIG. 2 is a schematic view one of a pair of antenna elements in anunwrapped state for the antenna shown in FIG. 1;

FIG. 3 is a schematic of an embodiment of this invention that producesan alternative to the antenna in FIG. 1;

FIGS. 4 and 5 are Smith charts for depicting calculated antennaimpedances; and

FIGS. 6A through 6L depict calculated gain comparisons of antennaperformance for an antenna constructed in accordance with this inventionand a standard antenna.

DESCRIPTION OF THE PREFERRED EMBODIMENT

In FIG. 1, a quadrifilar helix antenna 10, constructed in accordancewith this invention extends along a longitudinal axis 11. Four antennaelements 12, 13, 14 and 15 wrap helically about this longitudinal axis11 and extend from a feed or first end portion 16 to an unfed or secondend portion 17. The antenna element 12 and identical antenna elements13, 14 and 15 are wrapped as spaced helices about the axis 11. FIG. 1depicts the antenna elements 12 through 15 as being wrapped on a formfor facilitating an understanding of the antenna construction. This formcould be eliminated with the antenna elements being self-supporting.

Still referring to FIG. 1, an rf source 18 and a phase control 19 drivethe antenna 10 at a plurality of feedpoints 20 proximate the axis 11 atthe first end 16. A series of radially extending conductive paths 20A,20B, 20C and 20D couple the central feed points 20 to each of thehelically wrapped elements 12 through 15, respectively. The signalsapplied to these feedpoints are in phase quadrature. In one form, an RFsignal from the rf source 18 is applied to a 90° power splitter with adump port terminated in a characteristic impedance, Z₀. The two outputsof the 90° power splitter connect to the inputs of two 180° degree powersplitters thereby to provide the quadrature phase relationship among thesignals on adjacent ones of the antenna elements 12 through 15. It isknown that swapping the output cables of the 90° power splitter willcause the antenna to transfer between backfire and forward radiationmodes.

An antenna constructed in accordance with this invention achievespattern stability by making the antenna elements in FIG. 1 becomeelectrically shorter with increasing frequency without altering thephysical length of any of the antenna elements 12 through 15.Specifically, successive sections of the helix are shorted electricallyprogressively from the unfed end as frequency increases from the cut-infrequency by means of a capacitive shorting network. Obviously, a limitoccurs when the helix is so short as to no longer operate as a helix.

FIG. 2 depicts a pair of diametrically opposed antenna elements,specifically antenna elements 12 and 14 from FIG. 1. For clarity, theantenna elements 12 and 14 are shown in an unwound state. FIG. 2 depictsa number of capacitive elements connected between the antenna elements12 and 14 so that n−1 capacitive elements C₁ . . . C_(n−1) divide theantenna elements 12 and 14 into n segments. The pair of antenna elements13 and 15 include similar capacitive elements and the positions ofcorresponding capacitive elements in each pair will be the same.

Still referring to FIG. 2, the capacitive elements are evenlydistributed along the length of the element pair until reaching theradial feed sections 20A and 20C for the antenna elements 12 and 14. Thecapacitors decrease in value from the unfed end 17 to the feed end 16;that is:C₁>C₂> . . . >C_(n−1)>C_(n)  (1)With this relationship among the capacitive elements, the individualcapacitors at the unfed end 17 start to short out the helix at lowfrequencies. As frequency increases, the capacitive elements closer tothe feed point 20 start to short out the helix, thus effectivelyshortening the helix with frequency in a progressive fashion.

More specifically, following the principles for the frequencyindependent behavior with a log periodic dipole, the taper incapacitance values can be selected to vary logarithmically, so that thecapacitance of a given capacitor C_(i) is a constant multiple of thecapacitance of the preceding capacitor toward the unfed side 17,C_(i−1). That is, in equation form:C_(i)=τC_(i−1)  (2)where i is the capacitor number for 2≦i≦n−1 and τ is a constant.

In practice it has been found that it is easier to construct the antennaif each of the capacitive elements shown in FIG. 2 are formed by a pairof capacitors in series. FIG. 3 depicts the capacitive element thatwould replace the C₁ capacitor in FIG. 2 as including two capacitors,C_(1A) and C_(1B) in which:C_(1A)=C_(1B)=2C₁  (3)This facilitates the connection of two pairs of corresponding capacitiveelements to the two pairs of opposed antenna elements at the samerelative positions along the length of the antenna. In addition it hasbeen found that the range of capacitance values were specified byextreme values for the C₁ and C_(n−1) capacitors, and not by τ. Rather τwas determined by the capacitance values. The extreme case occurs if thecapacitor C₁ shorts the helix at the lowest frequency of operation,since the next few capacitors in sequence would be close to shorting outthe element resulting in a partial shorting of the antenna elements evenat the lowest operating frequency. Obviously, the shorting effect shouldonly occur at higher frequencies.

At the frequencies involved with such antennas, the wires connecting thecapacitors to the antenna elements and to each other have a finiteseries inductance that must be compensated. This compensation can beachieved by canceling the impedance with some or all of the impedancefor the capacitors connected to the wires.

For example, if a connecting wire has an effective physical length of 9″and a radius of 0.2388″, the wire will have an inductance of 1.633*10⁻⁷Henries. At an operating frequency of 200 MHz, the required capacitancefor canceling the wire impedance is 3.88 pF. Given the foregoingconsiderations, the value of C₁ must be less than 3.88 pF.

It has been found that the use of spaced capacitive shunts applied to aportion of the antenna can stabilize the pattern over a greaterbandwidth that can be achieved without the capacitive shunts. As aspecific example, capacitive shunts would improve an antenna having thefollowing characteristics:

Parameter Value Operating Mode Forward fire Unfed end impedance OpenInput impedance 200 ohms Helix cylinder diameter 9″ Cylinder length30.5″ Antenna element material Copper Antenna element diameter 0.2388″Number of segments N = 32 Frequency range 200-400 MHz Pitch angle 40°FIGS. 4 and 5 are calculated Smith charts that depict the variation ofinput impedance for the foregoing antenna without any shuntingcapacitive elements in FIG. 4 and with the addition of such shuntingcapacitive elements in FIG. 5 using a range of capacitors from C2=0.05pF to C10=0.025 pF that covered about one-third of the antenna startingproximate the unfed end 17. Each Smith chart is based upon the samecharacteristics impedance of Z₀=200 ohms and shows that the impedancedoes not vary significantly when these capacitive shunts are added tothe antenna, although FIG. 5 shows some loss of bandwidth especially atthe higher frequencies.

Each of FIGS. 6A through 6L depict the patterns produced by the antennawith and without shunting capacitive elements. In each, the solid line41 depicts the pattern for a conventional antenna; the dashed line 42,the pattern for the antenna modified in accordance with this invention.Each of FIGS. 6A through 6L is marked with the frequency for thepatterns.

There is little difference in performance up to 330 MHz, as shown inFIGS. 6A through 6E. That is, the patterns are essentially the same andstable with respect to different frequencies. As seen in FIG. 6F, theconventional antenna begins to generate multiple lobes at 43 as thepattern 41 begins to flatten and energy dissipating horizontally beginsto increase. The lobes 43 become progressively more pronounced as thefrequency increases as can be seen in FIGS. 6G through 6L. That is, theyare most pronounced in FIG. 6L. There is little indication of multiplelobes in patterns 42.

Below 340 MHz patterns 42 exhibit some flattening with frequency withrespect to the corresponding patterns 41. However, FIGS. 6F through 6Lshow that this difference ceases to exist above 340 MHz. The patterns 41in FIGS. 6K and 6L at 390 MHz and 400 MHz show the formation of nulls at44. No such nulls appear in patterns 42 at these frequencies.

Comparing at the patterns in FIGS. 6A through 6L, it will be apparentthat the shunting capacitors have stabilized the patterns 42 over thosepatterns 41 produced with a corresponding antenna without shuntingcapacitive elements. Moreover, these results are based upon an analysisof an antenna with a 40° pitch angle. Many quadrifilar helix antennasare constructed with greater pitch angles. At such greater angles, thenull effect shown in FIGS. 6K and 6L will be more pronounced and wouldbecome evident at lower frequencies. Thus, such antennas would benefitto even a greater degree from the capacitive shunting of this invention.Although there is some loss of impedance matching at higher frequenciesand some loss in the front-to-back ratios, the use of shuntingcapacitive elements will improve antenna performance where patternstability is a major consideration.

Thus, in accordance with this invention a quadrifilar helix antenna isprovided with a capacitive shunting network that electrically reducesthe length of antenna elements as operating frequency increases. As aresult, the energy radiates from the antenna with a pattern that isstable over a wide range of operating frequencies without the need ofphysical rearrangement of the antenna elements. While this antenna hasbeen depicted in terms of a specific capacitive shunting arrangement,including spacings and relative capacitance values, it will be apparentthat a number of different variations could also be included other thanthe structures shown in FIGS. 2 and 3. Consequently, it is the intent ofthe appended claims to cover all such variations and modifications ascome under the true spirit and scope of this invention.

1. An antenna for operating over a range of frequencies about a cut-infrequency, said antenna extending along an antenna axis between a feedend and an other end comprising;: a plurality of pairs of diametricallyopposed antenna elements wrapped helically about said axis from saidfeed end to said other end, each of said antenna elements having alength determined by the cut-in frequency; and capacitive means spanningsaid antenna elements in each said pair of antenna elements at apredetermined position from said other end for shorting said pairs ofantenna elements at a characteristic frequency greater than the cut-infrequency.
 2. An antenna as recited in claim 1 wherein said antennaincludes additional capacitive means spanning said antenna elements ineach said pair of antenna elements at other predetermined positionsalong the length of said antenna axis, each of said capacitive means atdifferent positions having a different capacitive impedance.
 3. Anantenna as recited in claim 2 wherein said different capacitive meanspositions are evenly spaced along said antenna axis.
 4. An antenna asrecited in claim 2 wherein said capacitive means most proximate to saidother end has a minimum capacitance and said capacitive means mostproximate to said feed end has a maximum capacitance.
 5. An antenna asrecited in claim 4 wherein the differences between the capacitance ofadjacent capacitive means are constant.
 6. An antenna as recited inclaim 4 wherein the differences between the capacitance of adjacentcapacitive means vary.
 7. An antenna as recited in claim 6 wherein thedifferences between the capacitance of the adjacent capacitive meansvary logarithmically.
 8. An antenna as recited in claim 4 wherein eachcapacitive means includes conductors for connection to said antennaelements and said capacitance of each said capacitive means isdetermined by the a first capacitance required to short circuit saidantenna elements at frequencies above the characteristic frequency forsaid capacitive means plus the a second capacitance required tocompensate inductance in said conductors.
 9. A quadrifilar helicalantenna for operating over a frequency bandwidth defined by a minimumoperating frequency and extending along an antenna axis between firstand second ends thereof, said antenna comprising;: four equiangularlyspaced helical antenna elements extending along said axis between saidfirst and second ends, each said antenna element having a length of atleast ¾ wavelength at a minimum antenna operating frequency and having asubstantially constant thickness and width, said antenna elementsconstituting first and second element pairs consisting of a pair ofdiametrically opposed antenna elements; and a plurality of sets ofcapacitive elements connected between said antenna elements in saiddiametrically opposed pairs, each said set being connected at adifferent position along the antenna axis and each capacitive element ina set connected to said respective antenna element pair at the sameposition along the antenna axis.
 10. A quadrifilar helical antenna asrecited in claim 9 wherein said antenna is divided into n axiallyextending segments and includes n−1 sets of axially spaced capacitiveelements.
 11. A quadrifilar helical antenna as recited in claim 10wherein said antenna is divided into n axially extending segments andincludes n−1 sets of evenly axially spaced capacitive elements.
 12. Aquadrifilar helical antenna as recited in claim 11 wherein eachcapacitive element has a capacitance that short circuits said antennaelements at a different frequency of a plurality of frequencies, andwherein each of which frequency in the plurality of frequencies isgreater than said minimum frequency.
 13. A quadrifilar helical antennaas recited in claim 12 wherein the a capacitance difference between thecapacitance in adjacent sets in the plurality of sets varies.
 14. Aquadrifilar helical antenna as recited in claim 12 wherein the acapacitance difference between the capacitance in adjacent sets in theplurality of sets varies logarithmically.
 15. A quadrifilar helicalantenna as recited in claim 12 wherein each capacitance element in theplurality of sets of capacitive elements includes at least one capacitorand conductors extending from said at least one capacitor to saidantenna elements, the capacitance of said capacitor being a function ofthe operating frequency at which said capacitance element shorts theattached antenna elements and the reactance of the conductors.
 16. Aquadrifilar helical antenna as recited in claim 15 wherein at least oneof said capacitance elements element in the plurality of sets ofcapacitive elements includes first and second capacitors in series. 17.A quadrifilar helical antenna as recited in claim 16 wherein the acapacitance difference between the capacitance in adjacent sets in theplurality of sets varies.
 18. A quadrifilar helical antenna as recitedin claim 16 wherein the a capacitance difference between the capacitancein adjacent sets in the plurality of sets varies logarithmically.
 19. Anantenna for operating over a range of frequencies about a cut-infrequency, said antenna extending along an antenna axis between a feedend and an unfed end, said antenna comprising: a plurality of pairs ofopposing antenna elements wrapped helically about said axis from saidfeed end to said unfed end, at least some of said antenna elementshaving a length determined, at least in part, from said cut-infrequency; and a plurality of capacitors coupled between antennaelements of said pairs of opposing antenna elements at locationsspanning said antenna elements, said capacitors being configured toshunt said antenna elements at a characteristic frequency greater thanthe cut-in frequency.
 20. The antenna as recited in claim 19, whereinsaid plurality of capacitors are coupled between said antenna elementsat predetermined positions along said antenna axis, and wherein saidcapacitors provide associated capacitive impedances.
 21. The antenna asrecited in claim 20, wherein said plurality of capacitors are connectedbetween said antenna elements at positions evenly spaced along saidantenna axis.
 22. The antenna as recited in claim 20, wherein a firstcapacitor most proximate to said unfed end comprises a minimumcapacitance among said plurality of capacitors, and wherein a secondcapacitor most proximate to said feed end comprises a maximumcapacitance among said plurality of capacitors.
 23. The antenna asrecited in claim 22, wherein capacitance differences between adjacentlycoupled capacitors in said plurality of capacitors are constant.
 24. Theantenna as recited in claim 22 wherein capacitance differences betweenadjacently coupled capacitors in said plurality of capacitors of vary.25. The antenna as recited in claim 24 wherein capacitance differencesbetween adjacently coupled capacitors in said plurality of capacitorsvary logarithmically.
 26. The antenna as recited in claim 22, whereinrespective capacitors in said plurality of capacitors compriseconductors for connection to said antenna elements, and whereinrespective capacitances of said capacitors is determined, at least inpart, by first predetermined capacitances required to short circuit saidantenna elements at frequencies above the characteristic frequency forsaid capacitors plus second predetermined capacitances to compensateinductance in said conductors.
 27. A broadband unidirectionalhemispherical coverage radio frequency antenna, comprising: a pluralityof helical antenna elements having a feed end for electricallyconnecting the plurality of elements; and a capacitor network configuredto capacitively shunt respective antenna elements in a pattern varyingas a function of distance from the feed end, to thereby control anapparent length of the antenna, wherein the apparent length of theantenna varies with frequency, and wherein the antenna is configured tooperate with a circular polarization and to exhibit a non-unityfront-to-back ratio over a range of electromagnetic frequencies.
 28. Abroadband unidirectional radio frequency antenna comprising: a pluralityof helically wound conductive antenna elements having a feed end; and anetwork of shunting capacitors, capacitively coupling at least two ofthe conductive antenna elements, at a plurality of locations distantfrom the feed end, said network of shunting capacitors configured toshort out an increasing number of the conductive antenna elements in theplurality of helically wound conductive antenna elements as frequencyincreases.
 29. A broadband unidirectional radio frequency antenna,comprising: a plurality of helically wound conductive antenna elements;and a capacitive network coupling the conductive antenna elements witheach other, said capacitive network configured to short out anincreasing number of the conductive antenna elements in the plurality ofhelically wound conductive antenna elements as frequency increases. 30.The antenna of claim 28, wherein the antenna is circularly polarized.31. The antenna of claim 28, wherein capacitance values of the networkof shunting capacitors decrease in value from an unfed end of theantenna elements to the feed end of the antenna elements.
 32. Theantenna of claim 31, wherein the capacitance values varylogarithmically.
 33. The antenna of claim 29, wherein the antenna iscircularly polarized, and wherein the antenna has a unidirectionalpattern, having a bandwidth of at least 200 MHz.
 34. The antenna ofclaim 29, wherein capacitance values of capacitors in the capacitivenetwork decrease in value from an unfed end of the antenna elements to afeed end of the antenna elements.
 35. The antenna of claim 34, whereinthe capacitance values vary logarithmically.